. 1996b . Treatment may be unreliable and can cause undesirable side effects. Preliminary
data indicate that resumption of cyclic ovarian activity after induced abortion by Ž
. cloprostenol is not common Ropstad et al., 1996a; Ropstad, 1998 .
5. Factors affecting reproduction
Traditionally, the basic idea behind reindeer herding has been to utilise the ability of these animals to grow and reproduce using pasture as their only food resource. As a
result, the productivity of reindeer herds is extremely vulnerable to changes in the environment with food limitation as the major threat. In this context, the degree of
access to nourishment in summer and shortage in winter and their interrelationship is of prime concern. The production potential of individuals is to a large extent determined by
the carrying capacity of the pastures, and by the composition of the herds and the culling policy. In this context, the fertility of animals kept through the winter and the survival of
Ž .
their offspring are the most important factors Lenvik, 1988a . 5.1. Population density
An increase in population density will usually cause a rapid reduction of winter range quality, and thus decrease the amount of food available per capita in winter. This will
cause decreases in reproduction rates and body mass of reindeer, and increases in Ž
mortality rates within the population Klein, 1968; Skogland 1983, 1985; Kojola et al., .
1995 , but it can also expose populations to the risk of crashes during extreme winter conditions.
5.2. Age The probability of pregnancy and a surviving autumn calf is related to maternal age
in a curvilinear manner. The probability of pregnancy in a semi-domestic herd in Finnmark, northern Norway, increased from 18.8 in yearlings to 81.8 in females
approaching 3 years of age. The highest probabilities of pregnancy were in the age Ž
. classes between 4 and 9 years around 90 . Older females showed a lower fertility
Ž .
unpublished data . These figures are slightly lower than those reported in wild Alaskan Ž
. Ž
caribou Adams, 1998
but in agreement with findings in Finland Eloranta and
. Ž
. Nieminen, 1991 and in wild Svalbard reindeer Stien et al., 1999 . There was also a
significant effect of age on the relationship between maternal body weight and calf Ž
. autumn weight Lenvik et al., 1988 . Within the same maternal weight classes, mean
calf autumn weight increased by 1.1 kgryear with increasing maternal age up to the age Ž
. of five years, after which there was no significant age effect Lenvik et al., 1988 .
5.3. Between-year Õariation Considerable variation in fecundity between years exists in various populations of
reindeer.
Ž .
Recent findings in Svalbard reindeer Rangifer tarandus plathrynchus indicate that in this particular subspecies, between-year variation in fecundity is mainly due to
Ž .
variation in foetal survival rather than variation in ovulation rate Stien et al., 1999 , suggesting that winter nutrition can be an important factor determining reproductive
Ž .
success in female reindeer and caribou Gates et al., 1986 . Low calf production follows harsh winters. On Nordenskjøldland on Svalbard calf production was only 5.9
Ž following severely restricted access to forage in late winter 1974 Alendal and Byrk-
. jedal, 1976 . In the same area the late winter pregnancy rate varied considerably
Ž .
Ž .
between 1979 and 1982 Tyler, 1987 and between 1996 and 1998 Stien et al., 1999 . In 1996 only 50 of the adult females were pregnant. Of those diagnosed as being
pregnant, 49 proved to be carrying a dead foetus. In 1998 the pregnancy rate was 90 Ž
. and only one female carried a dead foetus Ropstad et al., 1996b; Stien et al., 1999 .
No studies have as yet demonstrated any relationship between autumn weather conditions and pregnancy andror ovulation rate, although preliminary findings show a
negative correlation between precipitation in October and the probability of pregnancy Ž
. the following winter in a semi-domestic herd in Finnmark unpublished data . Within the
same herd winter pregnancy rate ranged between 62.6 and 82.8 in the period from 1992 to 1998. The probability of calf loss ranged between 7.9 and 22.6, and was
correlated positively with snow depth in April and negatively with May temperature.
5.4. Body weight Ž .
The meat production potential of a herd is mainly dependent on three factors: 1 Ž .
Ž . fecundity, 2 losses, and 3 the potential for growth in young animals. All these factors
are correlated with the body weight of sexually mature females in the herd. Studies in southern Norway and Finland showed that optimal criteria for calf production and
reproduction could not be met unless live body weight in females becoming pregnant
Ž .
exceeded 60 kg Lenvik 1988a; Eloranta and Nieminen, 1986 . Parturition rate is closely linked to autumn body weight, in general agreement with
previous reports of direct relationships between pregnancy or parturition rate and live or Ž
. Ž
dressed weight of female barren-ground caribou Daupine, 1976 , wild reindeer Reim- .
Ž .
ers, 1983a and semi-domestic reindeer Eloranta and Nieminen, 1986 in autumn. Similarly, pregnancy rates based on data collected at winter gatherings have been shown
Ž .
to vary directly with the intact or dressed weight of caribou Cameron et al., 1993 , Ž
. Ž
. semi-domestic reindeer Lenvik et al., 1988 and Svalbard reindeer Stien et al., 1999 .
Ž Body fat, rather than body weight itself, may influence breeding success Thomas, 1982;
. Ž
Adamczewski et al., 1987; Allaye-Chan, 1991 through an effect on ovulation Leader- .
Williams and Rosser, 1983; Ropstad et al., 1992 , although the metabolic mechanism is largely unknown.
Body condition at or near breeding may alter calving date via an effect on conception Ž
. time Baskin, 1970; Reimers, 1983b . Based on data from reindeer slaughtered approxi-
Ž .
mately four months after the rut, Lenvik 1988b obtained inverse correlations between Ž
. conception date and female dressed weight. However, Dauphine and McClure 1974
found no relationship between conception date and either body weight or fat content. The mechanism by which the timing of parturition varies is therefore uncertain. Results
Ž .
by Cameron et al. 1993 and several other studies suggest that the date of calving is mainly a function of the nutritional status of the pregnant female, but the possibility of
condition-related differences in the onset of ovulation cannot be discounted. Logically, female condition prior to the rut andror during late gestation could be implicated,
Ž .
depending on the timing of food limitation Skogland, 1984 . The consequences of late calving are potentially serious. Late-born calves have less
Ž opportunity for growth before the onset of winter Baskin, 1970; Bergerud, 1975;
. Skogland, 1983 and are therefore more likely to suffer mortality due to predation and
Ž .
undernutrition Dauphine and McClure, 1974; Bergerud, 1975 . Finally, females calving late might be unable to complete lactation in time to replenish body reserves by autumn,
Ž .
thereby reducing chances of conceiving that year Skogland, 1985 . 5.5. Calf surÕiÕal
Several reports show that maternal weight or food intake of Rangifer is positively Ž
correlated with calf birth weight Bergerud, 1975; Rognmo et al., 1983; Cameron et al., .
Ž 1993 and early survival Rognmo et al., 1983; Skogland, 1984; Eloranta and Nieminen,
. 1986; Adamczewski et al., 1987 . The effect of calf birth weight on survival apparently
extends beyond the perinatal stage. Domestic reindeer calves that survive to the end of Ž
the calving season are heavier at birth than those that die Eloranta and Nieminen, .
Ž .
Ž .
1986 . Low weight at birth may Espmark, 1980 or may not Rognmo et al., 1983 be sustained through summer, depending upon both milk production and the quality of the
Ž .
summer pasture White, 1991 . 5.6. PreÕious pregnancy
As indicated before, factors controlling the birth rate in a population of caribou are to a large extent of nutritional origin. The number of adult parturient females is to some
Ž extent dependent on the body condition of the animals the previous season Allaye-Chan,
. 1991; Cameron et al., 1993; Dauphine, 1976 , indicating that a decline in body condition
due to the demands of reproduction may result in reproductive pauses andror delayed breeding. The incidence of reproductive pauses has been reported to be between 11 to
Ž 33 in wild caribou populations with higher incidences in young animals Dauphine,
. 1976; Cameron, 1994 . In semi-domestic Norwegian reindeer, a lower percentage was
Ž .
found in adult females 4.8; unpublished results . Yearlings diagnosed as pregnant had a 73.9 chance of becoming pregnant the following year as compared to 89.5 in older
females.
5.7. Cohort effects Significant cohort effects in reindeer have not as yet been reported in the literature.
However, preliminary findings indicate that effects of early development on reproduc- tive performance in adulthood exist in reindeer and this effect might be related to
Ž .
nutritional status in late pregnancy unpublished results . The existence of pronounced differences in reproductive performance between successive cohorts has important
Ž implications not only for the understanding of the population biology of reindeer Albon
. et al., 1987 , but also for the understanding of environmental effects on the developmen-
tal competence of gonadal cells.
6. Conclusions
